1. Field of the Invention
The field of the current invention relates to optical communications and in particular to multiplexers/demultiplexers and their fabrication.
2. Description of the Related Art
The range of the use of optical communication has spread from trunk systems to metro systems and access systems in response to the recent demand for broadband services. In an access system in which subscribers are connected from a metro system, the wavelength multiplex system for the subscriber system has been introduced. In this wavelength multiplex system, an optical multiplexer/demultiplexer is required to multiplex and demultiplex the signal light.
FIG. 1 is a schematic diagram for explaining a conventional optical multiplexer/demultiplexer 208 using an optical fiber collimator. In FIG. 1, reference numerals 201, 202 designate optical fiber core wires of a 2-core optical fiber collimator 203, numeral 204 a collimator lens of the 2-core optical fiber collimator 203, numeral 205 a filter element, numeral 206 a focusing lens and numeral 107 an optical fiber core wire.
In the optical system of the optical multiplexer/demultiplexer shown in FIG. 1, the filter element 205 is arranged in the neighborhood of the focal point of the collimator lens 204. The light transmitted through and dispersed from the end surface of the optical fiber core wire 201 of the 2-core optical fiber collimator 203 is collimated by the collimator lens 204, and reflected on or transmitted through the filter element 205 arranged in the vicinity of the focal point of the collimator lens 204 in accordance with the wavelength thereof. The transmitted light is coupled to the optical fiber core wire 207 by the lens 206, and the reflected light, after being passed through the collimator lens 204 again, is collimated with the optical axis again and optically coupled to the other optical fiber core wire 202 of the 2-core optical fiber collimator 203. In the process, the accuracy of the position where the filter element 205 is located is estimated on the assumption that the numerical aperture (NA) of the optical fiber is 0.1, the center distance between the optical fiber core wire 201 and the optical fiber core wire 202 of the 2-core optical fiber collimator 204 is 125 μm and the focal length f of the collimator lens 204 is 1.8 mm. In the case where the error of the position of the filter element 205 is ±1 mm, the loss between the optical fiber core wires 201 and 202 is about 0.2 dB. This error is sufficiently small as compared with the thickness 10 to 20 μm of the portion of the filter element 205 other than the substrate. Therefore in an optical multiplexer/demultiplexer using a collimator lens, the positional accuracy of the filter element 205 has so far not been considered a problem. During production, the angle at which the filter element 205 is arranged has a larger effect on the reflection loss than the positional accuracy of the filter element 205.
With the spread of applications of the optical communication from the metro system to the access system, however, the optical multiplexer/demultiplexer in which the light emitted from the optical waveguide or the optical fiber enters the filter element without using a collimator lens has become increasingly important to reduce the cost and size of the system. In this optical multiplexer/demultiplexer, the light cannot be collimated by the collimator lens, and therefore a filter element is required to be arranged as near to the end surface of the optical fiber core wire as possible. For this reason, the filter element is required to be thin and generally formed of a fluoride polyimide filter (hereinafter referred to as the polyimide filter element) with a dielectric multilayer film for the filter arranged on the substrate of a polyimide fluoride film. An example of the features and the fabrication method of a fluoride polyimide filter element is described in detail in Japanese Unexamined Patent Publication No. 4-211203 (Patent No. 2608633) (hereinafter referred to as JP2608633). Further, optical multiplexer/demultiplexers used in the trunk system and the metro system are required to have high performance due to the narrow wavelength intervals involved.
FIG. 2 is a sectional view for explaining the optical multiplexer/demultiplexer 210 using no collimator lens. In FIG. 2, numerals 211, 212, 216 designate the cores of the optical waveguide, numerals 211a, 212a, 216a the center lines of the cores 211, 212, 216, respectively, numeral 213 a polyimide filter element using a polyimide fluoride film, numeral 213a a multilayer film having the filter function of the polyimide filter element 213, numeral 213b a substrate formed of a polyimide fluoride film, numeral 214 the surface of the multilayer film as an entrance surface of the optical multiplexer/demultiplexer 210, and numeral 217 an adhesive.
The polyimide filter element (polyimide fluoride filter) is described in detail in JP2608633, for example, and therefore not described in detail herein. The polyimide filter element is formed in such a manner that a provisional substrate of a material such as optical glass is formed with a polyimide fluoride film (5 μm thick, for example) having a comparatively small thermal expansion characteristic, on which a dielectric multilayer film is formed, after which the polyimide fluoride film, together with the dielectric multilayer film formed thereon, is separated from the provisional substrate thereby to produce the polyimide filter element having the polyimide fluoride film as a substrate. The whole of this polyimide filter element including the substrate, which can be reduced greatly in thickness and which can be bonded on the cut surface of an optical waveguide to form an optical multiplexer/demultiplexer, is expected to provide a promising optical part.
As an example of an optical waveguide with a filter, an optical waveguide using a wavelength select filter is described in “Polymer Optical Waveguide Design Architecture” co-authored by Hiroshi Masuda, Satoaki Shibata, Tatemi Ido and Makoto Takahasi, Hitachi Chemical Technical Report No. 39, pp. 37 to 40, published July 2002 (hereinafter referred to as the Masuda document). FIGS. 3 and 4 show an optical multiplexer/demultiplexer with a wavelength select filter inserted in the conventional optical waveguide described in the Masuda document. FIG. 3 shows an optical waveguide 230 in which a LPF (long-pass filter) 231 for reflecting the light of 1300 nm in wavelength and transmitting the light of 1550 nm in wavelength is used as a wavelength select filter. In FIG. 3, numerals 232 to 234 designate arrows for explaining the core of the optical waveguide 230 and numerals 235 to 238 the progression of the light. Also, FIG. 4 shows an optical waveguide 250 in which a SPF (short-pass filter) 251 for transmitting the light of 1300 nm in wavelength and reflecting the light of 1550 nm in wavelength is used as a wavelength select filter. In FIG. 4, numerals 252 to 254 designate the cores of the optical waveguide 250, and numerals 255 to 258 arrows for explaining the progression of light.
In the optical waveguide 230 shown in FIG. 3 using the LPF 231, the light having the wavelength of 1300 nm proceeding in the direction of arrow 235 through the core 232 is reflected on the LPF 231 and proceeds in the direction of arrow 236 through the core 233. Also, the light having the wavelength of 1550 nm proceeding in the direction of arrow 237 through the core 233 is transmitted through the LPF 231 and proceeds in the direction of arrow 238 through the core 234.
In the optical waveguide 250 shown in FIG. 4 using the LPF 251, on the other hand, the light having the wavelength of 1550 nm proceeding in the direction of arrow 255 through the core 252 is reflected on the SPF 251 and proceeds in the direction of arrow 256 through the core 253. Also, the light having the wavelength of 1300 nm proceeding in the direction of arrow 257 through the core 253 is transmitted through the SPF 251 and proceeds in the direction of arrow 258 through the core 254.
The filters used in the optical waveguide 230 shown in FIG. 3 and the optical waveguide 250 shown in FIG. 4 are each configured using a filter element (polyimide filter element) including a polyimide fluoride film as a substrate and having a thickness of 14 to 16 μm. In designing the optical waveguide, care is taken to offset the cores on both sides of the filter element taking the refractive index of the filter element into consideration. The Masuda document explains that the optical paths are offset as a condition to improve the performance of the optical multiplexer/demultiplexer, although the specifics of the offset are not described. Thus as an example, the offset by the refraction of light according to Snell's law is described below.
The shift, i.e. the offset of the optical path due to the change in refractive index is explained with reference to FIG. 5. FIG. 5 is an enlarged sectional view showing the polyimide filter element 530 inserted in the filter insertion section of the optical waveguide shown in FIGS. 3, 4. This polyimide filter element 530 (i.e. the LPF 231 or the SPF 251) is formed as a dielectric multilayer film including, alternately stacked, a layer having a thickness substantially equal to one fourth of the design reference wavelength with a comparatively high refractive index (hereinafter referred to as the high refractive index layer H) and a layer having a thickness substantially equal to one fourth of the design reference wavelength with a comparatively low refractive index (hereinafter referred to as the low refractive index layer L). The polyimide filter element 530 is actually formed by stacking the low refractive index layers L and the high refractive index layers H alternately on the substrate 533. However, for the convenience of explaining the offset of the light path and the behavior of the light in the polyimide filter element 530, the entire assembly of high refractive index layers H and of low refractive index layers L is illustrated as a separate and single layer designated in FIG. 5 by numerals 531 and 532, respectively. In FIG. 5, numeral 536 designates the incident light, numerals 537 to 540 the light paths of the incident light 536 entering and leaving the polyimide filter element 530, and numeral 536a a dotted line showing the extension of the incident light 536. Numeral 550 designates the entrance point of the incident light 536, numeral 535 the normal to the entrance surface 545 at the entrance point 550, character θ1 the angle between the incident light 536 and the normal 535, and characters θ2 to θ5 the angles that the light paths 537 to 540, respectively, form with the normal 535. Numeral 546 designates the exit surface of the light transmitted through the polyimide filter element 530, numeral 551 the exit point of the exit light, and numeral 552 an intersection between the dotted line 536a and the exit surface 546. Also, reference character d1 designates the distance between the exit point 551, at which the incident light 536 entering the polyimide filter element 530 from the entrance surface 545, and after refraction and proceeding through the filter element 530 exits from the exit surface 546 of the filter element 530 on the one hand and the intersection 552 between the exit surface 546 and the dotted line 536a on the other hand.
In the case where the polyimide filter element 530 is located in the optical waveguide shown in FIG. 5, according to Snell's law, the incident light 536 proceeds in the direction of arrow 547 and enters the polyimide filter element 530 at the entrance point 550, and while being refracted by the whole high refractive index layers H making up the filter element 530 then proceeds to the light path 537. The resulting light further proceeds to the light path 538, and while being refracted by the whole low refractive index layers L making the filter element 530, proceeds to the light path 539, exits from the exit point 551 of the exit surface 546 and proceeds in the direction of arrow 548 along the light path 540. On the other hand, assuming that the polyimide filter element 530 is absent, the incident light 536 proceeds linearly along the dotted line 536a, and from the intersection 552 between the dotted line 536a and the exit surface 546, exits in the direction of arrow 548. As a result, by inserting the polyimide filter element 530 in the optical waveguide, the exit point of the exit light 540 is shifted by the distance d1 on the exit surface 546. This is the offset of the light path caused by the change in the refractive index according to Snell's law.
By designing an optical multiplexer/demultiplexer having a filter element taking this offset of the light path into consideration, the loss of the light transmitted through the filter element can probably be reduced.
Nevertheless, the result of vigorous study made by the present inventor shows that the loss of the transmitted light cannot be sufficiently reduced nor can the problem of large variation in the loss of the transmitted light be solved by simply taking into consideration the light path offset due to refraction.